How Email Works?

Email communication is a complex process involving various servers and protocols to ensure that messages are sent, received, and managed securely and efficiently. Here's a step-by-step breakdown of how an email travels from a sender to a recipient.

Sender Mail Client​

• Imagine Shubh wants to send an email to his friend Parth. Shubh composes the email using his mail client, such as Outlook.

• Shubh's mail client submits the email to the Mail Submission Agent (MSA) on his outgoing mail server using the Simple Mail Transfer Protocol (SMTP).

Mail Submission Agent (MSA)​

• The MSA on Shubh's outgoing mail server receives the email from the mail client.

• The MSA authenticates Shubh's credentials (username and password) sent by the mail client. This is why logging into the mail client is necessary.

• After successful authentication, the MSA forwards the email to the Mail Transfer Agent (MTA).

Mail Transfer Agent (Sender MTA)​

• The sender's MTA takes over and looks up the recipient’s domain (mail.gmail.com) using DNS to find the IP address of the recipient's mail server.

• The MTA attempts to transfer the email to the recipient's mail server via SMTP.

• If the recipient’s server is unavailable, the MTA queues the email for periodic retry until it is successfully delivered. This non-blocking message queue ensures that emails are not lost even if there are temporary network issues.

Mail Transfer Agent (Receiver MTA)​

• The MTA has two primary functions:

  1. Sending emails to other MTAs.

  2. Sending emails to the local Mail Delivery Agent (MDA) if the recipient's domain matches the server's configuration.

• How does the MTA decide what to do? If the email is destined for a user within its domain (e.g., gmail.com), it sends the email to the MDA. Otherwise, it forwards the email to the appropriate external MTA.

Mail Delivery Agent (MDA)​

• The MDA performs tasks such as spam filtering to ensure unwanted emails do not reach the user's inbox.

• After filtering, the MDA delivers the email to Parth’s mailbox. Parth's mailbox is a specific directory or database entry within the server designated for his account.

• The mailbox might look something like this:

/parth
  inbox/
    - Email from Rahul (unread)
    - Email from Karan (unread)
    - Newsletter from Example Company (read)
    - Email with attachment (unread)
  sent/
    - Email to Rahul
    - Email to Karan
  drafts/
    - Unfinished email to Rahul

Receiver Mail Client​

• Parth wants to check his email, so he uses his email client (such as a web-based client like Gmail or a mobile app).

• Parth's email client connects to the IMAP server using his credentials (email address and password).

IMAP Server (Internet Message Access Protocol)​

• The IMAP server authenticates Parth's credentials to ensure he is authorized to access his mailbox.

• Once authenticated, the IMAP server grants Parth access to his mailbox.

• The IMAP server allows Parth’s mail client to fetch and manage emails stored in his mailbox, providing the capability to organize, read, and delete messages directly on the server.

• If Parth uses a mobile app, the email is downloaded and synchronized with his email client, so he can access his messages offline as well.